Line an 11x7 inch baking pan with parchment paper and set aside.
In a large bowl cream the butter and brown sugar until light and fluffy.
Add the eggs and vanilla and beat well.
Add the flour, rolled oats, baking powder, cinnamon and salt to the creamed mixture and beat to combine. The batter will be quite thick.
Stir in the apples and nuts (nuts are optional).
Spread batter evenly in the prepared pan.
Bake for 30 to 35 minutes until golden brown and center is almost firm.
Allow pan to cool completely on a wire rack. Dust top with powdered sugar.
When cooled, lift the squares out with the parchment paper and cut into 12 equal squares.
The calories shown are based on the cake being cut into 12 pieces, with 1 serving being 1 slice of cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information.**